This invention relates to a cryogenic refrigerator and, in particular, to a heat exchanger suitable for use in a cryogenic refrigerator utilizing the Gifford-McMahon refrigeration cycle.
A thermodynamic refrigeration cycle generally referred to as the Gifford-McMahon cycle, is disclosed in U.S. Pat. No. 2,906,101. A two-stage refrigerator utilizing this cycle is further described in U.S. Pat. No. 3,312,072 wherein a pair of different diameter cylinders are employed to process helium gas so as to attain extremely low temperatures. In this particular multiple stage embodiment, each cylinder slidably contains a displacer that is capable of reciprocating within the cylinder to vary the volume of an expansion chamber located at the bottom of the displacer. Initially, the refrigerant (helium gas) is compressed outside of the chamber to a higher pressure and is then cycled through the chamber to thermodynamically reduce the temperature of the working fluid down into the cryogenic region. The upper part of the displacer cylinders, however, remains relatively warm during the process. It has been noted that heat can build up in this critical region to a point where the seals acting between the displacer and the cylinder wall are harmed or destroyed. A heat build up in this critical region can also adversely effect the movement of the displacer within the cylinder. In the case of a multiple stage machine, that is a machine employing more than one displacer, the second stage in the machine is size limited by the amount of heat generated in this area. Typically, the maximum size of the last stage displacer element is only about one inch in diameter. This, in turn, seriously limits the capacity of multiple stage machines utilizing the Gifford-McMahon cycle.